In the past, photoelectric sensors, i.e., sensors that include one or more light-emitting devices, such as light-emitting diodes (LEDs), and one or more light-detecting devices, such as photodiodes, have not included a means to verify the operation of the sensing circuitry of the sensor. While undesirable, this lack is not particularly dangerous when photoelectric sensors are used to detect the presence or absence of objects on assembly lines and the like. However, as the use of photoelectric sensors has extended to other areas, such as the automatic guided vehicle (AGV) area, the inability to determine whether the sensing circuitry of photoelectric sensors is operating satisfactorily has become a more significant problem. The problem is of more concern because failure of the sensing circuitry of a photoelectric sensor in an AGV can result in damage to, or injury of, the associated mechanism, i.e., the AGV, and nearby items that the AGV can't "see".
This invention is directed to solving the foregoing problem by providing a method and apparatus for testing the operability of photoelectric sensor circuits. The nature of the invention is such that it can be readily "built into" a photoelectric sensor, either as part of an integral circuit that includes the photoelectric sensor circuitry, or as part of a separate test integrated circuit.